Many modern automotive ignition systems feature separate ignition coil assemblies for each cylinder of an engine. These assemblies have included an ignition coil, as well as interface and control electronics, typically in the form of silicon integrated circuits. Due to the fact that the assemblies are distributed around the engine, the assemblies are generally connected to an engine control module by a relatively lengthy wiring harness. Such wiring harnesses are susceptible to wiring insulation faults which can result in improper electronic spark timing (EST) signals being received by one or more of the assemblies, possibly resulting in excessively long “on” or “dwell” times. Excessively long dwell times can result in damage to or destruction of a coil or a coil current switching device of the assemblies.
To minimize damage to the assemblies, many modern ignition systems employ over-dwell protection timers to limit the maximum period of time that a coil can be energized for any given dwell event. Additionally, such systems may also employ a noise filtering timer function that prevents disruption of the protection timer circuitry for a short period of time after a timeout event. Such filtering is usually necessary due to the fact that when the ignition coil current is shut off at the end of the maximum allowed time a spark event may occur, producing significant amounts of radiated noise. Such noise can potentially reset dwell limiting circuitry, resulting in another timeout period being allowed to occur. This can repeat indefinitely, resulting in the same type of damage to the coil assembly that the over-dwell protection timer was intended to prevent. The noise filtering timer function, i.e., blinding timer circuitry, may be implemented using the same capacitor as is used for the maximum dwell timing by simply discharging the capacitor from its timeout reference voltage down to a lower reference voltage.
In order to optimize spark energy delivery it is desirable to have longer maximum dwell limitations at lower battery voltages when the coils charge more slowly due to the lower voltage. At higher battery voltages, the coils charge more quickly allowing for a reduction in the maximum allowed dwell time without limiting the amount of energy in the coil. Typical timer circuits compensate for different battery voltages by charging a capacitor in a controlled fashion until the voltage across the capacitor reaches a predetermined level. A basic technique of creating the desired battery voltage dependency in the over-dwell protection timer is to charge the capacitor by means of a resistor connected to a battery line.
Due to packaging area constraints and cost considerations, it is desirable to keep the timing capacitor value as low as possible, typically about 0.1 uF. For maximum dwell times of tens of milliseconds, the charging resistor will typically be several megaohms. With such a high impedance charging path, the charging currents are very small and any perturbations to this capacitor charging current can result in significant variations in the time needed to charge the capacitor to the reference level.
Due to cost considerations and the need to operate in the presence of high voltage transient noise impulses, the interface/control ICs are typically implemented using bipolar transistor processes. Because of the nature of bipolar transistors and their need for a finite amount of base drive current for operation, design of the circuitry that compares the capacitor voltage to the predefined reference voltage becomes problematic given the need to limit perturbations to the charging of the timing capacitor. Additionally, the blinding timer circuitry, which is tied to the timing capacitor, represents another possible source of error current.
What is needed is a technique for implementing dwell timing functions in such a manner as to limit error currents typically associated with bipolar comparators. It would also be desirable to allow for the inclusion of blinding timer functions, without additional perturbation of dwell times.